Glass is another brush-based entity. It's important that you understand that there is no specific entity class for glass. This is why in order to make glass, we use either a func_wall class brush-based entity or a func_breakable entity. The difference between these is that the func_breakable glass will....break!
To make glass, first select any glass texture. These usually have the word "glass" in them (yeah, really!). Now make a brush or brushes using that texture. Select the brush(es) that you want to magically transmogrify into glass, and click To Entity. Now go to the newly-made entity's properties. If you want to be able to bust the glass, make it a func_breakable. If you want the glass to be totally indestructible, make it a func_wall. If you made it a func_breakable, there are all kinds of attributes to tweak, like strength and material, but that's cosmetics, and u can worry about that later. What is important is the glass transparency, or how clear/blurry it is. Go to the Render Mode attribute, and set it to Texture. Now for FX Amount, enter a number corresponding to the transparency of your glass; 50-150 is good for Windex-clean glass. Remember that the glass brush has to have the glass texture in order for this to look right.
If you were to make a big block brush, and make it into a slab of glass using the above directions, you would notice something peculiar about the glass. When you look at a face of a glass brush, you cannot see any faces of the same brush that it is blocking. This makes it look like real glass, and not like a box of glass (which can be made with more than 1 brush if you so desire). What does this mean to the level designer? Make your glass brushes thin (after all, real glass tends to be pretty thin).
This article was originally published on 69th Vlatitude. TWHL only archives articles from defunct websites. For more information on TWHL's archiving efforts, please visit the TWHL Archiving Project page.